vec {"0 1 tab
or, simpler & more general,
vec {"_1 tab
-Dan
Please excuse typos; composed on a handheld device.
-----Original Message-----
From: Roman Odaisky <[email protected]>
Date: Sat, 22 May 2010 17:17:33
To: <[email protected]>
Subject: [Jprogramming] An element from each row of a table
Hi,
I’ve only recently gotten into J. It’s very interesting but I’m often unable
to find simple solutions to simple problems. I’m now struggling with finding a
way of taking a table and a vector and picking elements from the table with
positions in rows according to the vector, that is, the i-th element of the
vector specifies the element to be taken from the i-th row of the table. How
can this be done?
For example, given table =: 5 4 $ i. 20 and positions =: 0 2 0 1 2, the result
should be 0 6 8 13 8:
<0> 1 2 3
4 5 <6> 7
<8> 9 10 11
12<13>14 15
16 17<18>19
The best I was able to come up with is
pick =: {~ (<"1 @: ,.~ i.@:#)
table pick positions
0 6 8 13 18
which is rather ugly. I expected this to look more like “positions {"something
table” but nothing of that kind worked.
In Python that would look like this:
def pick(table, positions):
return [row[p] for row, p in zip(table, positions)]
or in a J-ish point-free way:
pick = compose(partial(starmap, getitem), zip)
It looks like no more than five verbs should be necessary. How can J do this?
--
TIA
Roman.
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